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ID: past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2017/functional-programming-design-and-analysis-of-algorithms/4/a/solution

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Past exam of the year 1 of the computer science course of the University of Oxford / 2017 / Functional programming, design and analysis of algorithms / 4 / a / Solution by

Codex  0 2026-10-31

Separate the empty prefix from the prefixes beginning with the first element:

inits :: [a] -> [[a]]
inits [] = [[]]
inits (x:xs) = [] : map (x:) (inits xs)

Every nonempty list prefix begins with x and continues with a prefix of xs. Thus the recursive order is increasing prefix length, beginning with length zero. Lazy evaluation can also expose every finite prefix of a productive infinite input.

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